Fix false positive in spec

This was passing because of a typo in the lambda. By specifying the
exception explicitly we ensure errors in test code don't mask errors in
the production code.
Don't raise missing file exceptions on build

Previously a guard against missing files prevented you from using this
library even if your app doesn't require the missing data. This patch
changes the behavior to lazily raise exceptions only when accessing
missing data.

The motivation here is to make it easier to work with data as provided
by transit centers. Although google's spec lists `calendar.txt` as a
required file, MTA does not include it with their LIRR data
(http://bit.ly/WETHPD).  Instead they manage everything through
`calendar_dates.txt`. This patch allows us to make use of the data as
provided even if it doesn't conform to spec.
fix require statements so they work on case-sensitive file system
add support for loading calendar dates file
yank debugger
Merge branch 'master' of github.com:nerdEd/gtfs
tidy up gem file and gemspec
fix gemspec to include correct files
update README w/ information about what's not
yet implemented
fix typo in README s/calendar/calendars
Merge pull request #5 from alwold/calendar-dates

Calendar dates
Merge pull request #3 from supaspoida/lazy_errors

Lazily raise exceptions for missing data
Clean up the parsing code, add previously unimplemented
parts of GTFS
fix typo in FareAttributes
add license to README
switch quotes in Rakefile
Allow LocalSource to be built from File object
add debugger to gemfile
bump version
small formatting update